1. Field of the Invention
This invention relates to waveguide hybrid couplers and to commutating hybrids and channel branching networks incorporating a plurality of such waveguide hybrid couplers therein. In particular, the waveguide hybrid coupler has a coupling region located at the intersection of two rectangular waveguides.
2. Description of the Prior Art
Various types of waveguide hybrid couplers are known in the prior art. Such couplers have four waveguide ports and have the property that signal power incident at one of the input ports will divide equally between two output ports with only a small fraction of the power escaping through the other port.
One well-known waveguide hybrid coupler is referred to as a magic "T" coupler and is arranged such that one of the four waveguides is perpendicular to the plane of the other three waveguides. Another hybrid coupler, referred to as a branch-line coupler, is described in a book titled "Microwave Filters, Impedance -- Matching Networks, and Coupling Structures" by Matthaei, Young and Jones published in 1964 by the McGraw-Hill Book Company at pages 809 et seq. The branch-line coupler is a directional coupler comprised of two parallel, rectangular waveguide sections which are coupled through a number of transverse branch lines. Such a coupler is also described in U.S. Pat. NO. 3,727,152 to J. Bodonyi. The branch-line coupler in that patent was fabricated by machining a pair of parallel waveguide paths in a planar block of material and then forming connecting channels or branch lines between the paths to obtain the required coupling.
A particular use for such hybrid couplers has been found in forming a commutating hybrid which is used to separate or combine frequency channels. The commutating hybrid is described in an article titled, "Channeling Filter for Trunk Waveguide Communication at Millimetric Wavelengths" by J. Bodonyi in the Marconi Review, Third Quarter 1973, at pages 160 to 192.
The commutating hybrid is formed by connecting two of the four ports of a first waveguide hybrid coupler to two of the four ports of a second waveguide hybrid coupler with two rectangular waveguide sections of different lengths. A pair of multiplexed channel frequencies presented to one of the unconnected ports of the first hybrid coupler will be separated by the commutating hybrid with one of the channel frequencies appearing at one of the unconnected ports of the second hybrid coupler and the other channel frequency appearing at the other unconnected port of the second hybrid coupler. The commutating hybrid is reversible in that separate frequencies applied to the unconnected ports of the second hybrid coupler will be combined by the commutating hybrid and appear at one of the unconnected ports of the first hybrid coupler. Such commutating hybrids may be connected in a cascade or tree arrangement to form a channel branching network to increase the number of frequency channels that may be combined or separated.
Although the magic "T" hybrid coupler provides the desired power division, it disadvantageously requires substantial space due to its nonplanar configuration and also is difficult and expensive to fabricate. Thus, where low cost and compactness of equipment is a necessity, the magic "T" configuration cannot be used. The branchline hybrid coupler described in the above-referred to Bodonyi patent also requires substantial space, for the dimensions of the branch lines are critical and the number of such lines dictate the size of such a coupler. Additionally, fabrication of such couplers is very expensive due to separate machining steps which are required to individually form the branch lines between the parallel waveguides.
A channel branching network, also described in the above-referred to Bodonyi patent, which incorporates a plurality of commutating hybrids which, in turn, is comprised of a pair of branch-line couplers obviously has the same space and machining problems. Accordingly, the fabrication of such a channel branching network is also time consuming and expensive.